Refine planner mode UI and error handling

Improve planner mode user experience with better error reporting,
cleaner tool output, and consistent log file placement.

- Propagate and display classified LLM errors to users with
  appropriate icons and context
- Display tool calls on single lines with truncated arguments
- Show LLM text responses without overwriting via UiWriter
- Ensure all logs write to workspace/logs directory consistently
- Set G3_WORKSPACE_PATH early in planning mode initialization

crates/g3-core/tests/todo_staleness_test.rs

@@ -53,7 +53,7 @@ impl UiWriter for MockUiWriter {
             .push(format!("STATUS: {}", message));
     }
     fn print_context_thinning(&self, _message: &str) {}
-    fn print_tool_header(&self, _tool_name: &str) {}
+    fn print_tool_header(&self, _tool_name: &str, _tool_args: Option<&serde_json::Value>) {}
     fn print_tool_arg(&self, _key: &str, _value: &str) {}
     fn print_tool_output_header(&self) {}
     fn update_tool_output_line(&self, _line: &str) {}